3 Answers
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There isn't a ton known about the virus in the Walking Dead. We know it spread fast, so it probably was airborne and it is also definitely spread through blood contact. We know it shows no symptoms in living humans, laying dormant until life functions cease in the host. We don't know who patient zero was or where the virus originated. The big thing we don't know is what the incubation period is and how long the infectious period is.

So a likely scenario for the disease is this: A high percentage of the population was already infected by the time the very first person who died after incubation. At that point, the epidemic is already spread worldwide, even though nobody has turned up symptomatic yet. So the very first day that people have had time to make through the incubation period and have enough of the virus in their bodies to turn into a walker, its way to late for any kind of quarantine to help. We don't see the ensuing chaos of the situation, but it would involve massive refugee movements and useless quarantines. Society can only take so much disruption before the entire thing grinds to a halt.

As for why now? Now is when the virus is around humans. All the plagues in human history have kicked off because a virus or bacteria has infected enough people to spread quickly through a population that had no defenses against it. Small pox was a terrible disease for Europeans but it was way worse for the Native Americans who had no antibodies to related diseases. The Walking Dead virus got a foothold in the population, spread with nobody being aware, and when the symptoms showed up, it was just too late to do anything about it.

Scientifically speaking, a particularly virulent pathogen could completely span the globe in a little as a week. If released in an airport of any major city with international travel, it could conceivably have been on every continent within a week and spread widely by the end of a month depending on its virulence and contagion level. The more contagious, the faster it spread. The more vectors, the more likely it spread. If it was a coordinated attack, it could have been placed at every major airport ensuring its spread in as little as 72 hours.

For the Walking Dead to make sense, we have to make certain suppositions:

The virus is likely a bio-engineered technology created by human beings. Judging from its behavior, it was designed to be a disruptive agent placed within an enemy population to destabilize the infrastructure of the target society. Designed to remain invisible until activated by an external force. Judging from Dr. Jenner's response, it was something completely new and unheard of before he began to work on it.

As a hidden agent, it would need to be completely invisible but able to be spread by contact or by aerosol dispersion. Disguised as a common influenza virus it could be spread by animals (think Bird Flu) or even people by coughing like any other influenza. In similar movies like the Crazies, the agent would have first been spread by plane and then later by contact with water supplies.

Since we don't know much of the history of the world before the outbreak, we cannot be sure if there was a massive influenza pandemic before the apocalypse occurred but if I were betting, I would assume the people responsible may have released it during that pandemic as a cover if the disease had very noticeable symptoms.

Here's where we run into problems. If it was asymptomatic, (meaning no one noticed they were infected), it might be more difficult to spread. It might require a deliver system like a water facility or an aerosol sprayer. This would make it difficult to spread easily.

If it was transferred by Patient Zero (first person believed infected during a pandemic, sought because they may reveal the original source of a natural pandemic) it would not have been able to spread as far asymptomatically without someone noticing it during testing. There have been people who were carriers of diseases without suffering from them, Typhoid Mary, for example.

This means the virus had to have been spread quickly, silently, artificially and at a pandemic speed for everyone, everywhere to have been infected without anyone being aware of it, which lends credence to the idea it was a bioweapon, spread intentionally or unintentionally is anyone's guess. Judging by the completeness of its distribution, I would assume terrorism as an intentional delivery.

Judging from the disease's current state in the remaining humans, it would appear to be an asymptomatic disease which opportunistically attacks the immune system of weaker organisms. The things needed to trigger the virus appear to be:

Catching a cold or other immune-depressing event: As long as you don't get too sick from something else (Andrea caught a cold but did not die thanks to Michonne care during the recent winter) the virus stays dormant,

Get an infection from a bite (Hershel got bit, but they cut off the infectious bitten region and prevent him from getting a dose of infection from the flesh of the long dead zombie) it isn't JUST the bite, its the level of necrotic flesh and disease in the mouth of the zombie. You get a disease from the bite-- sepsis, gangrene, tetanus and this causes the victim to begin to die and be overcome by the virus.

Taking an injury which wounds you past the point of first aid or medical support and you die (as in the case of Lori, who bled out in childbirth)

the virus is content to sit and wait for an opportunity to attack the host and transform it into a flesh-eating engine of destruction.

There may be other activation factors but touching the infected zombies, being splashed by their blood (assuming you aren't bathing in it) wearing their entrails as a disguise (didn't seem to cause anyone any difficulties in the early episodes) you will live to be a ripe old age, assuming one of your chronic infirmities (high blood pressure, diabetes), other diseases, or crazed friends don't get you caught and eaten first.

Open Dare! You have to leave the room and the group decides.....1.) A chemical warfare agent that got loose. Since it was bio-engineered in secret, the CDC was not aware of it in ample time to study it. With traveling from country to country being so common along with over population, it sread so rapidly that it was immpossible to stop.
2.) Maybe the CDC engineered it and it got loose. With the scientists who made it gone, no one is able to figure out an antidote so it became so rampid that we are where we we are.
3.) Someone bought their meat at Save-A-Lot.

Welcome to SFF.SE. This answer looks to be speculation. Unless you can provide a source for any of this, it would be better to post this as a comment. Once you have earned 50 reputation you will be able to comment everywhere.
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NullJan 29 at 19:16